In recent years, environmental problems have become an issue of great concern on a global scale. In the automotive industry, environmental protection measures during manufacturing processes have been actively promoted. In automotive manufacturing processes, reducing the amount of volatile organic compounds (VOC) released during coating processes has become a particularly urgent task.
Outer panels of automobile bodies are usually coated with a multilayer coating film including an undercoating film formed of a cationic electrodeposition coating composition, an intermediate coating film and a topcoating film, to impart corrosion resistance and aesthetic appeal. In order to reduce the amount of VOC, the use of aqueous intermediate coating and topcoating compositions has been promoted.
From the viewpoint of the film properties, curability, film surface smoothness, etc., aqueous two-package type clear coating compositions containing a polyisocyanate compound as a cross-linking agent have been studied as clear coating compositions for use as topcoating compositions. However, polyisocyanate compounds are generally hydrophobic and therefore have problematic insufficient dispersion stability in aqueous coating compositions.
A method is carried out in which a polyisocyanate compound, which is used as a cross-linking agent in an aqueous coating composition, is dispersed using a nonionic surfactant as a dispersant. However, because of the poor surface activating ability of nonionic surfactants, it is necessary to use a large amount of nonionic surfactant to stably disperse the polyisocyanate compound in water. As a result, the coating film has low hardness and is insufficient in film properties such as acid resistance.
Further, as a self-emulsifiable polyisocyanate compound for aqueous coating compositions, a modified polyisocyanate compound obtained by reacting a polyisocyanate compound with a hydrophilic surfactant having an active hydrogen group that can be reactive with an NCO group, is known.
Japanese Unexamined Patent Publication No. 1995-113005 discloses a polyisocyanate compound modified with a nonionic compound such as a polyalkylene ether alcohol or the like. The publication also discloses that the dispersion stability of a polyisocyanate can be improved by the introduction of a hydrophilic chain, such as a polyalkylene ether alcohol, into the polyisocyanate. The publication proposes an aqueous coating composition expected to have both the dispersion stability of the polyisocyanate in the aqueous coating composition and the stability of the NCO group.
However, when the polyisocyanate is modified with a nonionic compound having no polar group, the use of a compound with a long-chain nonionic group moiety is necessary in order to achieve sufficient water dispersion stability. Thus, the polyisocyanate compound has a soft structure, and therefore, the coating film obtained using the polyisocyanate compound as a cross-linking agent has reduced hardness, causing problematic insufficient film properties.
Japanese Unexamined Patent Publication No. 1995-113005 also discloses a polyisocyanate compound modified with an ionic compound such as a fatty acid salt, a sulfonic acid salt, a phosphoric acid ester, a sulfuric acid ester salt, or like anionic compound. However, the publication discloses that the use of an ionic compound often influences the reactivity of the isocyanate, as well as the coloration and deterioration of the coating film, and causes problems such as precipitation, agglomeration, etc., depending on the ionicity of the aqueous resin and the resin for use as the main component of the coating agent.
Japanese Unexamined Patent Publication No. 2003-533566 proposes an aqueous coating composition obtained using a sulfonic acid-modified polyisocyanate compound obtained by reacting a polyisocyanate compound with a specific sulfonic acid compound, so as to improve the dispersion stability in the aqueous coating composition.
However, coating compositions obtained using a polyisocyanate compound modified with an ionic compound, such as the above-mentioned aqueous coating composition, have problematic marked yellowing of the coating films due to overbaking, when the coating compositions are used as clear coating compositions in 2-coat 1-bake methods.
Further, aqueous two-package type clear coating compositions containing a polyisocyanate compound as a cross-linking agent have problems such as likely foaming of the coating films during curing, and insufficient finish appearance properties of the coating films, such as film surface smoothness and the like.
Japanese Unexamined Patent Publication No. 2000-506195 discloses a composition comprising a compound containing an anionic functional group and a polyethylene glycol chain fragment, and an isocyanate. The publication also discloses a two-package type coating composition comprising the isocyanate composition and an acrylic polymer containing an acidic group and a hydroxyl group. However, the coating film obtained in the publication is insufficient in foaming resistance and film surface smoothness.